The present invention relates to a solid-state image pickup element utilizing a photoelectric transducer.
Solid-state image pickup devices, typically CCD type solid-state image pickup devices, tend to be miniaturized and have high resolution in order to be mounted on portable electronics such as video cameras and electronic still cameras. In addition, to prevent sensitivity from being degraded by the miniaturization, it is necessary to bring the effective aperture area close to 100%; to meet this need, an on-chip microlens which collects incident light beams on the upper side of a photoelectric conversion element is adopted with a view to increasing the apparent light receiving area in the photoelectric conversion element.
On the other hand, in spite of such a contrivance, the S/N ratio, one of the most important determinants of the picture quality, is uniquely decided by the shot noise that the incident light itself has as a physical limitation. The S/N value in this case is proportional to the square root of p, which is the total number of incident photons that can be detected by a single pixel. Further, since p is proportional to the product of the time of accumulation and the number of incident photons per unit time, if p is sufficiently increased by exposure to light for a long time, the S/N ratio is naturally enhanced. That is, if the light receiving area is the same, the image pickup of an object with a small number of incident photons, in other words a dark object, is realized only by long hour exposure to light theoretically.
However, as it is also true of the NTSC system, which is one of the TV standard systems currently in use, the accumulation time per frame/field is fixed. Moreover, to secure a required level of dynamic resolution as well, varying the exposure time beyond a certain limit to match the relative brightness of the object is not realistic.
The object of the present invention is to provide a solid-state image pickup device and a photoelectric transducer for use therein capable of realizing an enhancement of the S/N ratio against shot noise by photons, taking account of the limit to the S/N ratio caused by the fact that light itself has fluctuations in the number of photons due to the Poisson distribution.
To achieve the object, one aspect of the present invention is a photoelectric transducer characterized by comprising a photoelectric converting means 1 for generating an electric charge according to incident light, an amplifying means 2 for amplifying a signal charge generated by photoelectric conversion, a memory for accumulating the signal charge amplified with a prescribed gain, and a consecutive gain control means 4 for controlling the gain of said amplifying means 2 according to the number of accumulated electric charges.
Another aspect of the present invention (corresponding to claim 2) is a photoelectric transducer comprising a first photoelectric converting section 12 for generating an electric charge according to incident light, an accumulating section for accumulating electric charges generated by said first photoelectric converting section 12, an amplifying section 14 for amplifying the electric charge (existing in 13) outputted from said accumulating section with a prescribed gain, and a storage section 16 for memorizing signal charges amplified by the amplifying section 14,
characterized in that said prescribed gain in the amplifying section is changed in 1 field/1 frame period, on the basis of the number of electric charges accumulated in said storage section 16 during a prescribed period before the time of application of that gain.
Still another aspect of the present invention is a photoelectric transducer comprising a first photoelectric converting section, disposed in a light receiving section, for generating an electric charge according to incident light, an accumulating section for accumulating signal electric charges from said first photoelectric converting section, and a negative feedback amplifying section,
characterized in that said negative feedback amplifying section is of a conducting type reverse to said first photoelectric converting section, disposed adjoining said first photoelectric converting section in the light receiving section, and includes a second photoelectric converting section for generating an electric charge according to incident light, and a charge source of the same conducting type as said photoelectric converting section for supplying electric charges to said accumulating section, and
signal charges flowing from said charge source is caused to be modulated, in 1 field/1 frame period, by the signal charges generated or accumulated by said second photoelectric converting section.
Yet another aspect of the present invention is a photoelectric transducer comprising a photoelectric converting section for generating an electric charge according to incident light and an accumulating section for accumulating the electric charges generated by said photoelectric converting section, characterized in that the part where photoelectric conversion is performed is an avalanche photodiode, the avalanche photodiode consists of a plurality of band offset type avalanche multiplication layer films stacked over said accumulating section via a plurality of conducting films connected to each other by fixed resistances, and these avalanche multiplication layer films vary in composition with reference the direction of stacking,
the multiplication rate of each avalanche multiplication layer film stacked varies, in 1 field /1 frame period, according to the number of electric charges accumulated in said accumulating section.
Still yet another aspect of the present invention is a photoelectric transducer comprising a photoelectric converting section for generating an electric charge according to incident light, an accumulating section for accumulating electric charges generated by said photoelectric converting section, and an A/D converter for digitizing the signal charges accumulated in said accumulating section at each breakdown or non-breakdown readout, characterized in that regarding each of signal charges consecutively outputted from the A/D converter, the signal charges having the largest value and the smallest value, or a plurality of prescribed signal charges are abandoned within a predetermined number of consecutive signal charges.
Generally, even if a signal charge that has been obtained by photoelectric conversion at a certain quantum efficiency is subjected to a certain output amplification by a multiplying means having a certain gain, an S/N ratio determined by the light shot noise never changes. On the other hand, according to the present invention, that multiplication gain in accordance with the hysteresis of the number of accumulated electric charges (the number of electric charges that have been accumulated so far). Thus it is so arranged the gain decrease if the number of electric charges that have been accumulated is large and, conversely, the gain be kept high if the number of accumulated electric charges is small. FIG. 23 is a schematic view showing that shot noise of light. It is seen that the number of photons is fluctuating. Accordingly, during each timing period, Mt to Mt+1, Mt+1 to Mt+2, . . . , the enhancement of the S/N ratio is intended by changing the gain in the intermediate and late phases during each period correspondingly to the number of photons in the early phase. FIG. 24 depicts the distribution of fluctuations, and the meaning of the present invention is to enhance the S/N ratio by cutting off the hatched parts.
As a result, in the average number of incident photons P (photons/s) for example, the shot-related S/N ratio that the number of photons coming incident in the accumulation period TF (s) during 1 field period has in itself is (P TF)1/2, but according to the present invention, it is possible to obtain an S/N ratio surpassing this value. FIG. 8 illustrates a simulative view in comparison with the S/N ratio (solid line) in one embodiment of the present invention and the S/N ratio (dotted line) in an ordinary according to the prior art. This point will be described in detail later.
Since the foregoing logic holds for image pickup devices for all kinds of electromagnetic waves including not only visible light but also infrared, ultraviolet and X rays, the present invention is applicable to these image pickup devices comprehensively.